Radar is used in many applications to detect target objects such as airplanes, military targets, vehicles, and pedestrians. Radar finds use in a number of applications associated with a motor vehicle such as for adaptive cruise control, collision warning, blind spot warning, lane change assist, parking assist and rear collision warning. Pulse radar and FMCW radar are conventionally used in such applications.
In a radar system, a local oscillator (LO) generates a transmit signal. A voltage controlled oscillator (VCO) converts a voltage variation into a corresponding frequency variation. The transmit signal is amplified and transmitted by one or more transmit units. In FMCW radar, the frequency of the transmit signal is varied linearly with time. For example, the frequency of the transmit signal may increase at a constant linear ramp rate from 77 GHz to 81 GHz in a period of about 100 microseconds. This transmit signal is referred as a ramp signal or a chirp signal. One or more obstacles scatters (or reflects) the transmit signal which is received by one or more receive units in the FMCW radar system.
A baseband signal is obtained from a mixer which mixes the transmitted LO signal and the received scattered signal that is termed an intermediate frequency (IF) signal. The IF signal is signal conditioned by a conditioning circuit which includes an amplifier and an anti-alias filter and is then sampled by an analog to digital converter (ADC) and processed by a processor to estimate a distance and a velocity of the one or more nearby obstacles. The frequency of the IF signal is proportional to the range (distance) of the obstacle(s).
Traditionally FMCW radar receivers use an in-phase (I) only receiver. However, an IQ receiver having an I channel and a quadrature-phase (Q) channel allows forming I and Q components of the received signals to generate an IF signal which includes both phase and amplitude data without a loss of information. This helps in improving the noise figure of the radar system compared to an I-only receiver by suppressing (to the extent of the image rejection ratio (IMRR)) fold-back of negative frequency components, including noise, and also helps keep the image band visible for external interference monitoring.
However, IQ imbalances are known to occur due to mismatches between the parallel sections (or channels) of the receiver chain providing the I signal path and the Q signal path. In FMCW radar the LO signal is a ramp signal, the same as the transmit signal, and a copy of that ramp signal is delayed (or advanced) by 90 degrees. When the direct LO output is mixed by a mixer with the original received signal, this produces the I signal, whereas when the 90° shifted LO output is mixed with the original received signal that produces the Q signal. In the analog domain, the delay is never exactly 90 degrees, and similarly the analog gain is never perfectly matched between the I signal path and the Q signal path.